[RE-wrenches] Imbalanced Legs in 3-Phase systems

jay peltz jay at asis.com
Fri Aug 8 21:55:04 PDT 2008


Hi Bill,

Can you explain why the 6kVA rule on split phase systems?

I mean why 6?

thanks,

jay

peltz power
On Aug 8, 2008, at 8:48 PM, Bill Brooks wrote:

> Jeff and Ryan,
>
> The 6kVA value comes from the California Rule 21 that I worked on.  
> It only
> applies to split-phase 240V systems (6kVA on 120V). There is no  
> specific
> limit for imbalance on a 3-phase service.
>
> A basic rule for single phase generation on a 3=phase service is to  
> rotate
> among the phases in a balanced manner. This means the largest  
> imbalance
> possible right now is 7kVA because of the SMA SB7000US. There is a
> single-phase SatCon that is 30kVA, but it would be silly to buy a
> single-phase Satcon when the major on 3-phase units.
>
> The best way to place the imbalanced single-phase generators on the  
> service
> is to test the current on all three phases and put the inverter(s)  
> on the
> most heavily loaded phase(s). This reduces the site imbalance by  
> reducing
> the imbalance on the serving transformer (lowering current on the most
> heavily loaded phases). It is common for 3-phase services to have  
> imbalances
> of 10 kVA or more, so single-phase inverters can reduce the problem  
> rather
> than make it worse.
>
> Make any sense?
>
> Bill.
>
>
>



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